In double acting hydraulic cylinders, such as shown in U.S. Pat. No. 3,673,925, fluid under pressure is alternately admitted under high pressure and exhausted under low pressure from opposite sides of a piston. The piston carries packings to prevent leakage of fluid from either side of the piston to the other. When U cups are employed for this purpose, two oppositely facing cups must be provided because U cups seal in only one direction and it is necessary to seal in both directions.
Heretofore it has been proposed to form a unitary packing assembly for double acting cylinders by placing two U cups of fabric material impregnated with hard synthetic rubber with their open ends facing each other and bonding the same to a spreader ring of softer synthetic rubber that fills the recesses in the U cups and provides a center section that keeps the U cups spaced apart.
Bonding is done by curing the rubber of all three parts simultaneously while the parts are in contact with each other in a mold. This results in a costly molding operation and is furthermore not suitable when it is desired to use a synthetic rubber material for the spreader ring and to use a relatively hard plastic material, such as polyurethane, for the U cups. In such case, to cure the rubber while assembled to the polyurethane rings will not result in a sufficiently strong bond between the parts.